Missing $ inserted issue

\usepackage[strings]{underscore}

The line number in the error message is the line of the .bbl file that is generated by bibtex, it is not the line of the .bib file. Therefore the question quotes the wrong lines. The line in the error message contains:

pp. 225--236, 10.1007/978-0-387-68772-8_18. [Online].

Therefore I think the entry in the .bib file contains

doi = {10.1007/978-0-387-68772-8_18},

It seems the DOI numbers are not well supported. The prefix doi: is missing:

doi:10.1007/978-0-387-68772-8_18

Or as URL:

http://dx.doi.org/10.1007/978-0-387-68772-8_18

Also special characters like _ are not well supported. The _ causes the trouble. Make a minimal example (MWE) that shows, how the doi entries in the .bib file are handled in your document.

If there are no special macros that handle the doi numbers, then you can try \_ instead of _ in the .bib file.

The problem is the underscore _ in a doi. It occurs in line 190 of the LaTeX file (or in the .bbl) not in the .bib file.

Search for 10.1007/978-0-387-68772-8_18 in your .bib file. You can use the url package and enclose the entry in \url{10.1007/978-0-387-68772-8_18} to avoid the error. Alternatively you can use a bibtex style that knows about dois.